Theme update screenshot are squished.

Description

In the update admin screen the theme screenshots are squished to 64px square. Too keep with the 4:3 aspect ratio of theme screenshots the image width should be 85px so no distortion to the image occurs.

Theme authors have a recommended screenshot size/aspect ratio but not all themes adhere to it, notably older ones. Those are going to get distorted a bit here, but not as badly as before. Otherwise, in order to maintain width and height in the layout (which is the desired effect), we'd have to add a wrapping container and CSS, like we do on Manage/Install Themes.